Pulmonary surfactant is a lipoprotein complex structure secreted by type II alveolar cells that have surface-active features. It consists of hydrophilic and hydrophobic sites. The primary function of surfactant is to reduce surface tension by adsorbing to the water-air division surface of the alveoli.
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- To increase lung compliance.
- Atelectasis prevention (collapse of the lung) at the end of exhalation.
- To restore collapsed airways
Secretion of surfactant
Secretion from cells can occur by constitutive and regulated pathways simultaneously. Constitutive secretion, in which substances are secreted continuously without prior concentration and storage, has not been studied in detail in type II cells.
It is possible, although unstudied, that constitutive, as well as regulated pathways, secrete one or more components of surfactant and that the two pathways may be utilized to differing extents during various states of health and disease. Studies in type II cells have focused on regulated secretion, in which a stimulus causes previously synthesized material stored in cytoplasmic organelles to release from the cell. In type II cells, regulated secretion occurs by classical exocytosis.
Extracellular stimuli modulate intracellular chemical events, which result in movement of lamellar bodies towards the apical surface of the type II cell, apposition of lamellar bodies to the apical plasma membrane, fusion of the limiting membrane of the lamellar bodies with the plasma membrane, and extrusion of lamellar body contents into the alveolus. It is likely that various intracellular second messengers, cytoskeletal elements, protein phosphorylation and dephosphorylation, and membrane fusion proteins are all involved in exocytosis in type II cells. The steps linking the initial stimulus to the eventual secretion of lamellar bodies are incompletely understood.